<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    手机号码：<input type="number"><button>发送</button>

    <script>
        var btn = document.querySelector('button'); 
        var time = 3;
        btn.addEventListener('click',function(){
            btn.disabled = true;
            var timer = setInterval(function(){
                if (time == 0) {
                    clearInterval(timer);
                    btn.disabled = false;
                    btn.innerHTML = '发送';
                    time = 3;//这个3需要重新开始
                }else{
                    btn.innerHTML = '还剩下'+time +'秒'
                    time--;
                }
               
                
            },1000)
        })
    </script>
</body>
</html>